[AMDGPU] Improve disassembler error handling

Summary:
llvm-objdump now disassembles unrecognised opcodes as data, using
the .long directive. We treat unrecognised opcodes as being 32 bit
values, so move along 4 bytes rather than the single byte which
previously resulted in a cascade of bogus disassembly following an
unrecognised opcode.

While no solution can always disassemble code that contains
embedded data correctly this provides a significant improvement.

The disassembler will now cope with an arbitrary length section
as it no longer truncates it to a multiple of 4 bytes, and will
use the .byte directive for trailing bytes.

Subscribers: arsenm, kzhuravl, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, llvm-commits

Differential Revision: https://reviews.llvm.org/D44685

llvm-svn: 328553